We propose to determine the structure of a complex between the E. coli chaperonin GroEL (Cpn60) and the gp 5 capsid protein of bacteriophage HK97. Suitable crystals have been obtained, they can be "flash frozen" at -160 degrees C and preliminary data have been collected to 3.6 angstroms resolution. Chaperonins, including members of the Cpn60 family are a ubiquitous class of proteins that assist in protein folding in vivo by binding to partially folded proteins; this prevents aggregation and may unfold "off pathway" intermediates and direct them back into the correct folding pathway. The gp 5 protein is a natural substrate of GroEL; it is the major capsid protein in the icosahedral head of the lambdoid bacteriophage HK97. The phase problem will be addressed by molecular replacement and/or isomorphous replacement using portions of the structure reported by the Sigler group. Synergy between these approaches is expected because the GroEL is composed of 14 identical subunits. Additional goals would be to cocrystallize GroEL with additional substrate proteins including the bacteriophage P22 capsid, mouse aldehyde dehydrogenase and proteolytic fragments of the HK97 capsid. These were chosen because they induce different electrophoretic band shifts in GroEL; this has been interpreted to suggest conformational differences among these complexes. All of these substrates would also be used in attempts to produce ternary complexes of the form substrate- GroEL-GroES. The proposed research would address structural questions including: Are the poorly ordered structural segments of the unliganded GroEL ordered in a substrate complex? If so, what is their structure? Are there hinge-like movements within the subunits? What is the degree of order exhibited by the bound substrate? Are the substrate binding surfaces of GroEL contiguous or disjoint: If the latter, does the substrate bind to more than one binding surface and if so how? What does this structure tell us about the broader issues of protein folding?